A need frequently arises in electronic circuits to detect when a clock signal changes to a particular state for a given amount of time. For example, a clock signal may be raised to a level (such as 5 volts) for a certain amount of time to indicate that a test sequence is about to begin, and a circuit is needed to measure that time.
An example of a detection circuit that has been developed for such needs is a capacitive charging circuit designed with a time constant suited to the needs of the application. In integrated electronic circuits, however, such capacitive charging circuits do not present attractive solutions because of the valuable space required to fabricate capacitors. For example, a circuit designed with a time constant of 625.times.10.sup.-6 seconds and using a 200 pF capacitor could require 500,000 .mu.m.sup.2 to fabricate the capacitor. Such sizes are unacceptable in integrated electronic circuits, where device density is a premier consideration.
Another technique that has been used avoids the penalty of large capacitors by varying the size of output logic devices coupled to relatively small capacitors. By this technique, output logic devices (such as metal oxide semiconductor field effect transistors, or MOSFETs) are sized so as to "turn on" or "turn off" at higher than typical voltages. For example, by increasing the width of the channel in a MOSFET, the transistor can be made to "turn on" at, for example, 4.5 volts, rather than a more typical 2.5 volts. By coupling such differently sized transistors to the output of relatively small capacitors, the time constant of the circuit is effectively increased, since an output signal will not be generated until the capacitor is charged high enough to trip the output transistors. This technique, however, is also unattractive, since it requires precisely-sized transistors for precise time constants, and accurately reproducing precisely the same size transistor from batch to batch, or process to process, is problematic. Moreover, variations in temperature and power supply will cause varying "turn on" and "turn off" voltages, resulting in varying time constants within the same circuit.
Therefore, a need has arisen for an integrated circuit design for detecting changes in a clock signal to static states with a relatively long time constant that maximizes device density by avoiding the use of large capacitors, and that provides a consistently reproduceable time constant.